Surfaces of many different materials are coated in a variety of applications for a wide range of motives, from aesthetic reasons to protection of the surface of the underlying substrate against physical and environmental damage. Often it is desirable to determine the thickness of these coatings, especially for applications where extremely thin coatings are critical on polymer or composite substrates containing directionally oriented inclusions, such as fiber-reinforced polymer composites.
Such directional orientation can cause undesirable variations in spectroscopic measurements, which ultimately can negatively impact measurements of thin coatings carried thereon.
In attempting to measure thicknesses of coatings, it has been found that small differences in orientation of the sensor with respect to the substrate can result in deleterious variations in spectral data. Thus, there is a need to eliminate the variation in spectral data due to differences in orientation of the sensor relative to the substrate and thus yield reproducible coating measurements that are independent of the sensor with respect to the substrate.
Most known nondestructive coating measurement techniques are limited in their applicability for polymer composite and other non-metal substrates. In one known method, ultrasound testing is used to determine thickness of coatings on surfaces of composite or plastic substrates. However, ultrasound testing has been shown to have subjective data interpretation issues and therefore can be inconsistent and unreliable. Moreover, ultrasound techniques have great difficulty in determining coating thicknesses below approximately 0.002 inches (2 mils). In another known method, the same sensor and substrate alignment is maintained for all measurements. This is difficult to achieve, especially in cases where the coating is opaque. With this method, reproducibility is difficult because small variations in orientation negatively impact reproducibility of spectroscopic data. Many of the known nondestructive methods do not correct for orientation effects. Current methods to compensate for orientation effects are based on anecdotal information and include making measurements at several orientations and using the average result.
Near infrared (NIR) spectroscopy has been successfully used to measure coating thicknesses on a variety of substrate materials. However, to date, there has been significant difficulty in using NIR methods for high specular reflectance coatings, coatings of less than 2 mils, and coatings on oriented substrates. One problem encountered in obtaining accurate and reproducible thickness measurements is specular reflectance from the coating that interferes with the NIR diffuse reflectance measurements. The disclosure of the co-pending, commonly-owned application having U.S. Ser. No. 11/566,902, filed Dec. 5, 2006 proposes the use of a diffuser element between the light source and the substrate on which the coating thickness is being measured. The diffuser described in the above-mentioned disclosure somewhat reduced variation due to orientation differences, but did not eliminate them.
However, in attempting to measure opaque coatings (for example, paint, sealants, protective films, etc.) on substrates containing directionally oriented components, including fiber-reinforced polymer composites, the substrate orientation is not observable.
Therefore, there is an unmet need to provide an apparatus and method to accurately and nondestructively measure the thickness of thin coatings applied to substrates containing directional oriented components, such as fiber-reinforced polymer composites, without regard for relative orientation of the sensor or the substrate.